An ultrasonic diagnostic apparatus is used to make a noninvasive checkup on a subject by irradiating him or her with an ultrasonic wave and analyzing the information contained in its reflected wave (i.e., echo). For example, a conventional ultrasonic diagnostic apparatus that has been used extensively converts the intensity of the reflected wave into its associated pixel luminance, thereby presenting the subject's structure as a tomographic image. In this manner, the internal structure of the subject can be known.
Meanwhile, some people are attempting recently to track the motion of a subject's tissue more precisely and evaluate the strain and the elasticity, viscosity or any other physical (attribute) property of the tissue (among other things, an arterial vascular wall) mainly by analyzing the phase of the reflected wave (Patent Document No. 1 or 2).
If the elastic property of each site of the arterial vascular wall under inspection is presented as a two-dimensional distribution by using such a technique, a portion of the arterial vascular wall with a unique elastic modulus can be easily identified. That is why an ultrasonic diagnostic apparatus could be used in diagnosing arterial sclerosis.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 10-5226        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2000-229078        Patent Document No. 3: Pamphlet of PCT International Application Publication No. 2004/112568        Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 2000-271117        